Tinting over existing tint

My front windows have 35% tint on them already. I want to get them down to 15% or somewhere around there. Would I need to put 50% on it to make that happen. Just want to make sure before I order the tint and it not be dark enough or be too dark.

Good call on all- If there is flim in there already it WILL fail at some point. If you tint over it with new flim the new stuff will be good for longer- but will fall off when the old falls off and you will be redoing it all over again.

I've heard soda water works well to get the old adhesive off...and I'll be trying in my scion sometime in the near future.